Modifications of epoxy resins have been widely studied in an attempt to broaden their application. Although epoxy resins are excellent in electrical characteristics, adhesion, and thermal characteristics, they are generally brittle and liable to suffer from cracking due to stress-strain, heat or dynamic impact on curing or use. In order to offset these disadvantages and to endow epoxy resins with toughness, use of a hardener having a long chain aliphatic group or rubbery properties, addition of a compound having elastomeric properties, addition of a plasticizer, e.g., asphalt substances and glycols, addition of reinforcing fibers, e.g., glass fiber, aramid fiber and carbon fiber, and introduction to an epoxy compound a molecular group exhibiting rubber elasticity have been proposed as disclosed in Mitsukazu Ochi, KOBUNSHI, Vol. 38, No. 3, p. 200 (1989).
However, these modificaitons failed to obtain sufficient effects because an epoxy resin and some modifiers added are not sufficiently compatible with each other, or some modifications impair excellent characteristics of epoxy resins, such as heat resistance and adhesion, or some modified epoxy resins are not suitable for practical use because of greatly increased cost.